1. Field of the Invention
This invention relates to apparatus for the processing of semiconductor wafers. More particularly, this invention relates to a clamping ring apparatus for yieldably engaging the upper surface of a semiconductor wafer peripherally adjacent the outer edge of the wafer to clamp the wafer to an underlying support pedestal.
2. Description of the Related Art
In the plasma etching of semiconductor wafers, it is conventional to cool the wafer (or more properly to maintain it at a preselected temperature) during the etch process by flowing a gas, e.g., helium or argon, through an opening in the underlying cathode support pedestal to the space between the top surface of the pedestal and the undersurface (backside) of the wafer. The presence of such a gas in this space serves t& enhance the thermal coupling of the wafer to the underlying pedestal which serves to cool the wafer, i.e., to maintain it at a preselected temperature despite heat generated by the plasma etching being carried out on the top surface of the wafer.
While the use of such a thermal coupling gas serves the intended purpose of cooling the wafer by thermally coupling it to the underlying pedestal (which, in turn, may be maintained at a preselected temperature by a heater or circulation of cooling fluid through the pedestal) to avoid excessive and therefore deleterious heat buildup during the etching process, escape of the thermal coupling gas into the etch chamber from the edges of the wafer/pedestal may interfere with the etch process by changing the makeup of the etch gases flowing into the chamber.
It is, therefore, customary to secure the wafer to the pedestal by a clamping ring actuated by a clamping mechanism which exerts the necessary force needed to clamp the wafer to the underlying pedestal. Such prior art clamping rings are usually either continuous rings which engage the periphery of the top surface of the wafer, i.e., adjacent the end edge of the circular wafer, or intermittent fingers which peripherally engage portions of the top surf ace of the wafer to urge it against the underlying pedestal.
The gas such as helium or argon supplied to the space between the backside of the wafer and the upper surface of the pedestal is supplied under pressure (relative to the pressure within the etch chamber) which serves to deflect or warp the thin wafer into a dome shape. It is not unusual for the underlying surface of the pedestal to therefore also be provided in such a dome shape to compensate for this expected behavior of the wafer, thereby providing even spacing between the backside surface of the wafer and the underlying pedestal.
However, this combination of thermal coupling gas pressure and wafer deflection also often results in an uneven surf ace interface between the wafer and the pedestal adjacent the end edge of the wafer, resulting in undesirable leaking of the helium or argon into the etch chamber, despite the use of a clamping ring or clamping fingers.
To avoid such leaks, it has become the customary practice to place an elastomer seal between the pedestal and the wafer. The seal complies with the wafer surface and more or less effectively reduces the leak.
However, there are certain drawbacks or penalties which accompany the use of an elastomer seal in a vacuum chamber,, including particle generation, plasma cleaning restrictions (the gases used may react with the elastomer), and temperature restrictions. The latter may not only be a problem with regard to etch process temperatures employed, but also due to the wafer temperature when it enters the etch chamber or zone. In integrated processing, the wafer may be at a temperature as high as 400.degree. C. from prior processing. Placing such a hot wafer on an elastomer seal can result in many undesirable effects, not the least of which is partial melting of the seal, resulting in the wafer sticking to the pedestal.
It would, therefore, be desirable to provide apparatus for clamping a semiconductor wafer to an underlying pedestal in a manner which provides satisfactory peripheral sealing of the backside of the wafer, adjacent the end edge of the wafer, to the underlying pedestal, without the use of elastomeric sealing materials, to permit a thermal coupling gas to flow under pressure (with respect to the pressure in the etch chamber) into the space between the backside of the wafer and the underlying pedestal surface without any substantial escape of the gas into the etch chamber.